Determining media device activation based on frequency response analysis

ABSTRACT

Methods, apparatus, systems and articles of manufacture (e.g., physical storage media) to determine media device activation based on frequency response analysis are disclosed. Example methods disclosed herein include reusing first frequency values of audio sensed with a microphone to determine a first frequency response of the sensed audio, the first frequency values having been determined to perform watermark detection during a first monitoring time interval. Disclosed example methods also include comparing the first frequency response to a reference frequency response to determine whether the media device was active during the first monitoring time interval.

RELATED APPLICATION(S)

This patent arises from a continuation of U.S. patent application Ser.No. 14/542,155 (now U.S. Pat. No. 9,747,906), which is entitled“DETERMINING MEDIA DEVICE ACTIVATION BASED ON FREQUENCY RESPONSEANALYSIS,” and which was filed on Nov. 14, 2014. U.S. patent applicationSer. No. 14/542,155 is hereby incorporated by reference in its entirety.Priority to U.S. patent application Ser. No. 14/542,155 is herebyexpressly claimed.

FIELD OF THE DISCLOSURE

This disclosure relates generally to media device monitoring and, moreparticularly, to determining media device activation based on frequencyresponse analysis.

BACKGROUND

Media monitoring systems typically include one or more device meters tomonitor media presented by one or more media devices located at one ormore monitored sites. In some examples, the device meters employed bythe media monitoring systems use watermarks (also referred to as codes)decoded from the presented media, and/or signatures (also referred to asmedia fingerprints or just fingerprints) generated from the presentedmedia, or both, to monitor (e.g., identify and/or track) the media beingpresented by the monitored media devices. Some media monitoring systemsfurther employ media device activation detectors (also referred to ason/off detectors) to detect whether the monitored media devices areactive (e.g., turned on) or inactive (e.g., turned off) to verifywhether the decoded watermarks and/or generated signatures actuallycorrespond to media presented by the media devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example monitored site including anexample device meter implementing an example device activation detectorto detect whether an example monitored media device is active based onfrequency response analysis as disclosed herein.

FIG. 2 is a block diagram illustrating an example implementation of theexample device meter of FIG. 1 in communication with an example dataprocessing facility.

FIG. 3 is a block diagram illustrating an example implementation of theexample device activation detector included in the example device meterof FIGS. 1 and/or 2.

FIG. 4 is a block diagram of an example frequency response determinerthat may be used to implement the example device activation detector ofFIGS. 1, 2 and/or 3.

FIGS. 5A-B illustrated example frequency responses determined by theexample frequency response determiner of FIG. 4.

FIG. 6 is a flowchart representative of example machine readableinstructions that may be executed to implement the example device meterof FIGS. 1 and/or 2.

FIG. 7 is a flowchart representative of example machine readableinstructions that may be executed to implement the example deviceactivation detector of FIGS. 1, 2 and/or 3.

FIG. 8 a flowchart representative of example machine readableinstructions that may be executed to perform further processing in theexample device activation detector of FIGS. 1, 2 and/or 3 when amonitored media device is determined to be active (e.g., turned on).

FIG. 9 is a flowchart representative of example machine readableinstructions that may be executed to implement the example frequencyresponse determiner of FIG. 4.

FIG. 10 is a block diagram of an example processor platform structuredto execute the example machine readable instructions of FIG. 6 toimplement the example device meter of FIGS. 1 and/or 2.

FIG. 11 is a block diagram of an example processor platform structuredto execute the example machine readable instructions of FIGS. 7 and/or 8to implement the example device activation detector of FIGS. 1, 2 and/or3.

FIG. 12 is a block diagram of an example processor platform structuredto execute the example machine readable instructions of FIG. 8 toimplement the example frequency response determiner of FIG. 4.

Wherever possible, the same reference numbers will be used throughoutthe drawing(s) and accompanying written description to refer to the sameor like parts, elements, etc.

DETAILED DESCRIPTION

Methods, apparatus, systems and articles of manufacture (e.g., physicalstorage media) to determine media device activation based on frequencyresponse analysis are disclosed. Example methods disclosed hereininclude determining a reference frequency response based on firstfrequency values of an audio signal used to perform watermark detectionfor a first time interval during which a media device has beendetermined to be active. Such example methods also include determining asecond frequency response based on second frequency values of the audiosignal used to perform watermark detection for a second time intervaldifferent from the first time interval. Such example methods furtherinclude comparing the second frequency response with the referencefrequency response to determine whether the media device was activeduring the second time interval.

In some disclosed example methods, comparing the second frequencyresponse with the reference frequency response includes determining avalue of a dot product of the second frequency response and thereference frequency response. Some such example methods also includecomparing the value of the dot product to a threshold to determinewhether the media device was active during the second time interval.

Additionally or alternatively, some disclosed example methods furtherinclude, in response to determining that a valid watermark was detectedwhen performing watermark detection for the first time interval usingthe first frequency values, determining that the media device was activeduring the first time interval.

Additionally or alternatively, some disclosed example methods furtherinclude determining a third frequency response based on third frequencyvalues of the audio signal used to perform watermark detection for athird time interval after the first time interval. Some such examplemethods also include, in response to determining that a valid watermarkwas detected when performing watermark detection for the third timeinterval, replacing the reference frequency response with the thirdfrequency response. Some such example methods further include comparingthe third frequency response with the reference frequency response(e.g., prior to replacing the reference frequency response with thethird frequency response), and, in response to determining that thethird frequency response does not match the reference frequencyresponse, indicating that at least one of: (i) the media device movedrelative to a position of the media device during the first timeinterval, or (ii) an environment in which the media device is inoperation has changed relative to an arrangement of the environmentduring the first time interval.

Additionally or alternatively, in some disclosed example methods,comparing the second frequency response with the reference frequencyresponse is performed in response to determining that no valid watermarkwas detected when performing watermark detection for the second timeinterval. Also, some such disclosed example methods further include,when no valid watermark was detected when performing watermark detectionfor the second time interval, conditioning whether to determine asignature from a portion of the audio signal corresponding to the secondtime interval based on whether the comparing of the second frequencyresponse with the reference frequency response indicates that the mediadevice was active during the second time interval.

Additionally or alternatively, some disclosed example methods furtherinclude conserving processing resources by disabling further generationof monitoring data using a portion of the audio signal corresponding tothe second time interval in response to determining that, based on thecomparing of the second frequency response with the reference frequencyresponse, the media device was inactive during the second time interval.

These and other example methods, apparatus, systems and articles ofmanufacture (e.g., physical storage media) to determine media deviceactivation based on frequency response analysis are disclosed in furtherdetail below.

As noted above, some prior media monitoring systems employ media deviceactivation detectors (also referred to as on/off detectors) to detectwhether monitored media devices are active (e.g., turned on) or inactive(e.g., turned off). By determining whether a monitored media device isactive or inactive, such media monitoring systems can verify whetherdecoded watermarks and/or generated signatures actually correspond tomedia presented by a monitored media device. However, in prior mediamonitoring systems, such media device activation detectors are typicallyseparate from the device meters employed to decode watermarks and/orgenerate signatures from the media presented by the monitored mediadevices, or are included in the device meters but use processingresources in addition to the processing resources already employed toperform watermark decoding and/or signature detection.

In contrast with such prior media device activation detectors, mediadevice activation detection based on frequency response analysis, asdisclosed herein, is able to reuse frequency values of an audio signaldetermined when performing watermark detection on the audio signal. Byreusing the frequency values that were already determined whenperforming watermark detection, media device activation detection basedon frequency response analysis as disclosed herein is able to reduce theamount of a device meter's processing resources utilized to determinewhether a monitored media device is active. Furthermore, unlike priormedia device activation detectors, which may be limited to determiningwhether a monitored media device is active, media device activationdetection based on frequency response analysis as disclosed herein isalso able to ascertain whether the monitored media device has movedrelative to a prior position, or whether an environment in which themedia device is operating has changed, in addition to determiningwhether the monitored media device is active. Moreover, in at least someexamples, media device activation detection based on frequency responseanalysis as disclosed herein can conserve processing resources andreduce power consumption by disabling further generation of monitoringdata during a monitoring interval in response to determining that, basedon the frequency response analysis, the media device was inactive duringthe monitoring interval. These and other potential benefits associatedmedia device activation detection based on frequency response analysisare described in further detail below.

Turning to the figures, a block diagram of an example monitored site 100including an example device meter 105 implementing an example deviceactivation detector 110 to detect whether an example monitored mediadevice 115 is active based on frequency response analysis, as disclosedherein, is illustrated in FIG. 1. In the illustrated example of FIG. 1,the device meter 105 includes an example microphone 120 to sense audioin the monitored site 100 and to produce a resulting sensed audio signalfor processing by the device meter 105. The example microphone 120 maybe implemented by any number and/or type(s) of microphones, acousticsensors, transducers, etc. As described in further detail below, thesensed audio signal output from the microphone 120 is processed by thedevice meter 105 to monitor media presented by the media device 115. Inthe illustrated example, the media monitored by the device meter 105 cancorrespond to any type of media presentable by the media device 115. Forexample, monitored media can correspond to media content, such atelevision programs, radio programs, movies, etc., as well ascommercials, advertisements, etc. In the illustrated example, the devicemeter 105 determines metering data that may identify and/or be used toidentify media presented by the media device 115 (and, thus, infer mediaexposure) at the monitored site 100.

In the illustrated example, the media device 115 monitored by the devicemeter 105 can correspond to any type of audio, video and/or multimediapresentation device capable of presenting media audibly and/or visually.For example, the media device 115 can correspond to a television and/ordisplay device that supports the National Television Standards Committee(NTSC) standard, the Phase Alternating Line (PAL) standard, the SystèmeÉlectronique pour Couleur avec Mémoire (SECAM) standard, a standarddeveloped by the Advanced Television Systems Committee (ATSC), such ashigh definition television (HDTV), a standard developed by the DigitalVideo Broadcasting (DVB) Project, etc. As other examples, the mediadevice 115 can correspond to a multimedia computer system, a personaldigital assistant, a cellular/mobile smartphone, a radio, a tabletcomputer, etc.

When the media device 115 in the example monitored site 100 is active(e.g., turned on), the sensed audio signal obtained by the microphone120 will likely include audio emanating from one or more speakers of themedia device 115, such as the example speaker(s) 125. Furthermore, in atypical environment such as the example monitored site 100, the audioemanating from the speaker(s) 125 of the media device 115 may take oneor more example acoustic paths 130 to reach the microphone 120. Suchexample acoustic paths 130 may include a direct acoustic path and/or oneor more indirect paths resulting from the audio emanating from thespeaker(s) 125 and reflecting off of one or more walls, such as theexample walls 135, and/or reflecting off of one or more objects locatedin the example monitored site 100, such as the example object 140illustrated in FIG. 1. The different acoustic paths taken by the audiofrom the speaker(s) 125 of the media device 115 to the microphone 120cause multipath interference in the audio sensed by the microphone 120.Due to this multipath interference, some of the frequency components inthe frequency spectrum of the sensed audio signal obtained by themicrophone 120 may undergo constructive interference and be amplifiedrelative to other frequency components having similar acoustic spectralenergies at the point of emission (e.g., at the speaker(s) 125).Additionally or alternatively, other frequency components in thefrequency spectrum of the sensed audio signal obtained by the microphone120 may undergo destructive interference and be attenuated relative toother frequency components having similar acoustic spectral energies atthe point of emission (e.g., at the speaker(s) 125).

For a given source of audio in the example monitored site 100, such asthe speaker(s) 125 of the media device 115, the multipath interferenceeffects (e.g., the pattern of frequency component amplifications and/orattenuations) observed by the microphone 120, which is assumed to be ata fixed location in the illustrated example, yield a sensed audio signalthat was subjected to an acoustic frequency response (e.g., the patternof frequency component amplifications and/or attenuations) that is afunction of the source location (e.g., the location of the speaker(s)125), the location of the microphone 120, the acoustics of the monitoredsite 100 (e.g., the arrangement of the objects 140, walls 135, etc. inthe room corresponding to the monitored site 100), etc. If thesecharacteristics of the monitored site 100 remain constant, the resultingfrequency response may be assumed to characterize the audio emanatingfrom the source location (e.g., the location of the speaker(s) 125).Other audio sources, such as people in the monitored site 100 conversingwith each other, other media devices, appliances, etc., included in themonitored site 100, etc., will likely produce different acousticfrequency responses at the microphone 120.

Accordingly, the device meter 105 of the illustrated example includesthe example device activation detector 110 to determine whether themedia device 115 is active based on analyzing the acoustic frequencyresponse exhibited by the audio signal sensed by the microphone 120. Asdisclosed in further detail below, the example device activationdetector 110 reuses frequency values of the sensed audio signal, whichare determined by the meter 105 when performing watermark detection fora given monitoring time interval, to further determine a frequencyresponse exhibited by the sensed audio signal. The device activationdetector 110 of the illustrated example then compares the determinedfrequency response for the given monitoring time interval with areference frequency response indicative of the media device 115 beingactive to determine whether the media device 115 was active during thegiven monitoring time interval. The example device activation detector110 then uses the result of determining whether the media device 115 wasactive to further control operation of the example meter 105. Exampleimplementations of the meter 105 and the device activation detector 110are illustrated in FIGS. 2 and 3, respectively, which are described infurther detail below.

Although one example media device 115 is depicted in the illustratedexample of FIG. 1, media device activation detection based on frequencyresponse analysis, as disclosed herein, is not limited thereto. Instead,media device activation detection based on frequency response analysis,as disclosed herein, can be used to monitor multiple different mediadevices included in a monitored site, such as the example monitored site100. For example, the device activation detector 110 can be structuredto determine different reference frequency responses indicative ofdifferent respective media devices being active in the monitored site100. Such a device activation detector 110 can then be structured tocompare a determined (e.g., measured) frequency response for a givenmonitoring time interval to the different reference frequency responsesto determine which, if any, of the different respective media deviceswere active during the monitoring time interval.

A block diagram of an example implementation of the meter 105 of FIG. 1is illustrated in FIG. 2. As noted above, the example meter 105 of FIG.2 determines metering data that may identify and/or be used to identifymedia presented by a monitored media device, such as the example mediadevice 115, at a monitored site, such as the example monitored site 100.Additionally, the example meter 105 of FIG. 2 includes the exampledevice activation detector 110 to determine whether the media device 115is active based on analyzing the acoustic frequency response exhibitedby the audio signal sensed by the microphone 120.

In the illustrated example of FIG. 2, the meter 105 includes an exampleaudio signal sampler 205 to sample the audio signal sensed by themicrophone 120. The audio signal sampler 205 samples the audio signaloutput from the microphone 120 at a given sampling rate, such as asampling rate of 8 kHz, 16 kHz, 44.1 kHz, or some other value, andperforms analog-to-digital conversion to generate audio data from thesensed audio signal. In some examples, the audio signal sampler 205performs other processing, such as filtering, automatic gain control,etc., on the sensed audio signal output from the microphone 120 and/orthe digital audio samples obtained from the sensed audio signal.

The example meter 105 of FIG. 2 also includes an example watermarkdetector 210 and an example signature generator 215 to determinemetering data from the audio signal sensed by the microphone 120. Morespecifically, the watermark detector 210 of the illustrated exampleprocesses the audio data obtained by the audio signal sampler 205 todecode watermarks embedded in and/or otherwise included with an audioportion of media presented by a monitored media device, such as themedia device 115. The signature generator 215 of the illustrated examplealso processes the audio data obtained by the audio signal sampler 205,but generate signatures based on the audio portion of the mediapresented by the monitored media device, such as the media device 115.

In the context of media monitoring, watermarks may be transmitted withinmedia signals. For example, watermarks can be used to transmit data(e.g., such as identification codes, ancillary codes, etc.) with media(e.g., inserted into the audio, video, or metadata stream of media) touniquely identify broadcasters and/or media (e.g., content oradvertisements), and/or to convey other information. Watermarks aretypically extracted using a decoding operation.

In contrast, signatures are a representation of some characteristic ofthe media signal (e.g., a characteristic of the frequency spectrum ofthe signal). Signatures can be thought of as fingerprints. Signaturesare typically not dependent upon insertion of identification codes(e.g., watermarks) in the media, but instead preferably reflect aninherent characteristic of the media and/or the signal transporting themedia. For example, signature-based media monitoring techniquesgenerally use one or more inherent characteristics of the monitoredmedia during a signature sampling interval to generate a substantiallyunique proxy for the media. Such a proxy is referred to as a signatureor media fingerprint, and can take the form of a series of bits, datavalues, a waveform, etc., representative of the media signal(s) (e.g.,an audio signal and/or a video signal) forming the media presentationbeing monitored. A good signature is usually one that is repeatable whenprocessing the same media presentation, but that is unique relative toother (e.g., different) presentations of other (e.g., different) media.Systems to utilize codes (e.g., watermarks) and/or signatures for mediamonitoring are long known. See, for example, Thomas, U.S. Pat. No.5,481,294, which is hereby incorporated by reference in its entirety.

In the illustrated example of FIG. 2, the watermark detector 210performs one or more decoding procedures to detect watermarks embeddedin the audio data obtained by the audio signal sampler 205. For example,the watermark detector 210 may be structured to process the audio dataobtained by the audio signal sampler 205 to detect watermarks encoded inone or more frequencies of the sensed audio signal, or otherwise encodedin the frequency domain of the sensed audio signal. Examples ofwatermarks encoded in the frequency domain of an audio signal and thatcan be detected using the example watermark detector 310 of FIG. 3include, but are not limited to, examples described in U.S. Pat. No.8,359,205, entitled “Methods and Apparatus to Perform Audio Watermarkingand Watermark Detection and Extraction,” which issued on Jan. 22, 2013,U.S. Pat. No. 8,369,972, entitled “Methods and Apparatus to PerformAudio Watermarking Detection and Extraction,” which issued on Feb. 5,2013, and U.S. Publication No. 2010/0223062, entitled “Methods andApparatus to Perform Audio Watermarking and Watermark Detection andExtraction,” which was published on Sep. 2, 2010, all of which arehereby incorporated by reference in their entireties. U.S. Pat. Nos.8,359,205, 8,369,972 and U.S. Publication No. 2010/0223062 describeexample watermarking systems in which a watermark is included in anaudio signal by manipulating a set of frequencies of the audio signal.

In some examples, the example watermark detector 210 of FIG. 2 may alsobe structured to process the audio data obtained by the audio signalsampler 205 to detect watermarks encoded in one or more time domaincharacteristics of the sensed audio signal, such as by modulating theamplitude and/or phase of the audio signal in the time domain. Examplesof watermarks encoded in the time domain of an audio signal and that canbe detected using the example watermark detector 210 include, but arenot limited to, examples in which spread spectrum techniques are used toinclude a watermark in an audio signal. For example, such a watermarkcan be encoded in the audio signal by (1) spreading the watermark bymodulating the watermark with a pseudo-noise sequence and then (2)combining the spread watermark with the audio signal. Detection of sucha watermark involves correlating the audio signal (after beingwatermarked) with the pseudo-noise sequence, which de-spreads thewatermark, thereby permitting the watermark to be detected after thecorrelation.

In the illustrated example of FIG. 2, the signature generator 215performs one or more signature generation procedures to generatesignatures from one or more characteristics of the audio data obtainedby the audio signal sampler 205. In some examples, the signaturegenerator 215 processes the audio data obtained by the audio signalsampler 205 to generate a sequence of signatures at a signaturegeneration time interval (e.g., corresponding to a new signature beinggenerated at intervals of 1 second, 2 second, or any other interval)and/or based on occurrence of one or more events. Each site signaturegenerated by the signature generator 215 is intended to berepresentative of a respective segment of the media (e.g., correspondingto 1 second, 2 seconds, or some other duration of the media) beingpresented by the monitored media device, such as the example mediadevice 115. Examples of signature techniques that can be implemented bythe signature generator 215 include, but are not limited to, any or allof the techniques described in U.S. Pat. No. 4,677,466 issued to Lert etal. on Jun. 30, 1987; U.S. Pat. No. 5,481,294 issued to Thomas et al. onJan. 2, 1996; U.S. Pat. No. 7,460,684 issued to Srinivasan on Dec. 2,2008; U.S. Publication No. 2005/0232411 to Srinivasan et al. publishedon Oct. 20, 2005; U.S. Publication No. 2006/0153296 to Deng published onJul. 13, 2006; U.S. Publication No. 2006/0184961 to Lee et al. publishedon Aug. 17, 2006; U.S. Publication No. 2006/0195861 to Lee published onAug. 31, 2006; U.S. Publication No. 2007/0274537 to Srinivasan publishedon Nov. 29, 2007; U.S. Publication No. 2008/0091288 to Srinivasanpublished on Apr. 17, 2008; and U.S. Publication No. 2008/0276265 toTopchy et al. published on Nov. 6, 2008, all of which are herebyincorporated by reference in their respective entireties.

In the illustrated example of FIG. 2, the device meter 105 includes theexample device activation detector 110 to determine whether a monitoredmedia device, such as the media device 115, is active based on analyzingthe acoustic frequency response exhibited by the audio signal sensed bythe microphone 120. In some examples, the device activation detector 110could be structured to determine the acoustic frequency response of thesensed audio signal by implementing a calibration procedure inconjunction with the monitored media device. For example, the monitoredmedia device could be configured to generate tones at a set of differentfrequencies, such as tones spaced at 100 Hz intervals in the range of 1kHz to 5 kHz, or some other arrangement of frequencies. Assuming theenergies of the tones are the same, the device activation detector 110could then determine the variations in the energies of the tones in thesensed audio signal to determine the frequency response exhibited by theaudio signal sensed by the microphone 120. The resulting frequencyresponse could then be used as a reference frequency response indicativeof the monitored media device being active (e.g., turned on), whichcould be compared to subsequent frequency responses of the sensed audiosignal determined by the device activation detector 110 during operationof the meter 105 to perform media monitoring.

However, such a calibration procedure may be impractical as it wouldtypically involve (1) adapting the media monitored device to generatethe calibration tones, (2) providing a mechanism to cause the mediamonitored device and the meter 105 to invoke the calibration procedure,and (3) utilizing additional processing resources of the meter 105 toenable the device activation detector 110 to process the receivedcalibration tones. Accordingly, to avoid the foregoing drawbacks andreduce consumption of processing resources, the device activationdetector 110 of the illustrated example determines the frequencyresponse exhibited by the audio signal sensed by the microphone 120based on an example procedure that reuses frequency values alreadydetermined by the example watermark detector 210 when attempting todecode watermarks in the sensed audio signal. The device activationdetector 110 of the illustrated example further uses the presence orabsence of valid watermarks detected by the watermark detector 210 todetermine whether a frequency response of the sensed audio signaldetermined by the device activation detector 110 for a given monitoringtime interval corresponds to a reference frequency response indicativeof the monitored media device being active (e.g., turned on), or is ameasured frequency response for which the operational status of themonitored media device is unknown and, therefore, is to be ascertained.

An example implementation of the device activation detector 110 of FIG.2 is illustrated in FIGS. 3 and 4, which are described in further detailbelow. Example frequency responses that may be determined by the exampledevice activation detector 110 are depicted in FIGS. 5A-B, which aredescribed in further detail below.

In the illustrated example of FIG. 2, the device activation detector 110verifies whether watermarks decoded by the watermark detector 210 and/orsignatures generated by the signature generator 215 during a givenmonitoring time interval actually correspond to media presented by themonitored media device, such as the media device 115. For example, ifthe device activation detector 110 determines that the monitored mediadevice was active (e.g., turned on) during the monitoring time interval,then it is likely that any decoded watermarks and/or generatedsignatures are representative of the media being presented by themonitored media device. As such, the device activity determination madeby the device activation detector 110 (e.g., which indicates that themonitored device is active in this case) may be used to confirm thevalidity of the decoded watermarks and/or generated signaturescorresponding to the monitoring time interval. Conversely, if the deviceactivation detector 110 determines that the monitored media device wasinactive (e.g., turned off) during the monitoring time interval, then itis likely that any decoded watermarks and/or generated signatures arenot representative of the media being presented by the monitored mediadevice but, instead, are representative of some other audio sourceand/or correspond to false detections. As such, the device activitydetermination made by the device activation detector 110 (e.g., whichindicates that the monitored device is inactive in this case) may beused to invalidate (e.g., discard) the decoded watermarks and/orgenerated signatures corresponding to the monitoring time interval.

In some examples, the device activation detector 110 further controlsoperation of the example signature generator 215 to conserve processingresources by disabling further generation of monitoring data during agiven monitoring interval in response to determining that, based on itsfrequency response analysis, the media device was inactive during themonitoring interval. For example, the meter 105 of FIG. 2 may bestructured to invoke the signature generator 215 only when validwatermarks are not detected by the example watermark detector 210 for agiven monitoring time interval. In such examples, the device activitydetermination made by the device activation detector 110 may furthercondition whether the signature generator 215 is to be invoked when novalid watermarks were detected for the given monitoring time interval.For example, if the frequency response analysis performed by the deviceactivation detector 110 indicates that the monitored media device wasinactive during the given monitoring time interval and no validwatermarks were detected by the watermark detector 210, then the deviceactivation detector 110 may prevent the signature generator 215 fromgenerating signatures to conserve processing resources. Otherwise, if novalid watermarks were detected by the watermark detector 210, but thefrequency response analysis performed by the device activation detector110 indicates that the monitored media device was active during thegiven monitoring time interval, then the device activation detector 110may enable (e.g., permit) the signature generator 215 to generatesignatures for use in monitoring the media presented by the (presumably)active media device.

In some examples, the device activation detector 110 additionally oralternatively provides status information related to the monitored site,such as the example monitored site 100, being monitored by the examplemeter 105. For example, such status information may include the deviceactivation determination made by the device activation detector 110based on its frequency response analysis. Additionally or alternatively,in some examples, the device activation detector 110 provides otherstatus information. For example, as mentioned above, the deviceactivation detector 110 determines that a frequency response determinedfor audio signal sensed by the microphone 120 is a reference frequencyresponse indicative of a monitored media device being active if validwatermark(s) are detected by the watermark detector 210 for the givenmonitoring time interval. In a subsequent monitoring time interval, thedevice activation detector 110 determines a new, measured frequencyresponse corresponding to the subsequent time interval. If validwatermarks are detected by the watermark detector 210 for thissubsequent monitoring time interval, the device activation detector 110determines that the new, measured frequency response is also indicativeof the monitored media device being active (e.g., turned on). In someexamples, the device activation detector 110 also compares the new,measured frequency response with the prior reference frequency response.If the two frequency responses are determined to match (as describedbelow), then the device activation detector 110 indicates that theenvironment of the monitored site (e.g., the monitored site 100) isunchanged.

However, if the two frequency responses do not match, then the deviceactivation detector 110 indicates that a change in the environment ofthe monitored site has occurred between the prior monitoring intervaland the subsequent interval. For example, the change can correspond tothe monitored media device having moved relative to a position of themedia device during the prior monitoring time interval, or anarrangement of the environment (such as a position of the microphone120, position(s) of the room object(s) 140, etc.) has changed relativeto the prior monitoring time interval, etc.

In some examples, the device activation detector 110 further checks thesignal strength (and/or or some other characteristic(s)) of the audiosignal sensed by the microphone 120 when valid watermarks are detectedbut the reference and measured frequency responses do not match. Forexample, if the monitored media device (e.g., the media device 115) isinactive, the meter 105 might detect valid watermarks in an audio signalemanating from a media device at another location (e.g., another room).However, the audio signal from this other media device would typicallyhave lower energy than an audio signal received from the monitored mediadevice. Therefore, when valid watermarks are detected but the referenceand measured frequency responses do not match, in some examples thedevice activation detector 110 checks that the signal strength of thesensed audio signal satisfies (e.g., meets or exceeds) a thresholdbefore determining that the prior reference frequency response should bereplaced with the new measured frequency response because theenvironment has changed. Otherwise, if the signal strength of the sensedaudio signal does not (e.g., meets or exceeds) satisfy the threshold, insuch examples device activation detector 110 determines that themonitored media device is not the source of the sensed signal, and maydiscards the watermarks and measured frequency response determined forthe monitored time interval being processed.

The example device meter 105 includes an example data reporter 220 toreceive watermarks decoded by the watermark detector 210, signaturesgenerated by the signature generator 215, device activity determinationsand/or other status information provided by the device activationdetector 110, and/or any other information determined by the devicemeter 105. The data reporter 220 stores some or all of the foregoinginformation as metering data, and reports this metering data via anexample network 225 to an example data processing facility 230. The dataprocessing facility 230 performs any appropriate post-processing of themetering data to, for example, determine audience ratings information,identify targeted advertising to be provided to the monitored site 100,etc. In the illustrated example, the network 225 can correspond to anytype(s) and/or number of wired and/or wireless data networks, or anycombination thereof.

A block diagram of an example implementation of the device activationdetector 110 of FIGS. 1 and/or 2 is illustrated in FIG. 3. The exampledevice activation detector 110 of FIG. 3 includes an example frequencyresponse determiner 305 to determine frequency responses of a sensedaudio signal (e.g., the audio signal sensed by the microphone 120) fordifferent monitoring time intervals based on an example procedure thatreuses frequency values already determined for the sensed audio signalby a watermark detector, such as the example watermark detector 210,when performing watermark decoding for the respective monitoring timeintervals. For example, the frequency response determiner 305 maydetermine (1) a first frequency response based on first frequency valuesof the sensed audio signal used by the watermark detector 210 to performwatermark detection for a first monitoring time interval, (2) a secondfrequency response based on second frequency values of the sensed audiosignal used by the watermark detector 210 to perform watermark detectionfor a second monitoring time interval, (3) a third frequency responsebased on third frequency values of the sensed audio signal used by thewatermark detector 210 to perform watermark detection for a thirdmonitoring time interval, etc. An example implementation of thefrequency response determiner 305, which implements an example procedurethat reuses frequency values already determined for the sensed audiosignal by the example watermark detector to determine frequencyresponses of the sensed audio signal for different monitoring intervals,is illustrated in FIG. 4 and described in further detail below.

The example device activation detector 110 of FIG. 3 also includes anexample reference response identifier 310 to identify whether a measuredfrequency response determined by the frequency response determiner 305corresponds to a reference frequency response indicative of a monitoredmedia device, such as the media device 115, being active (e.g., turnedon). For example, the reference response identifier 310 can bestructured to determine that a monitored media device, such as the mediadevice 115, was active during a first monitoring time interval inresponse to determining that one or more valid watermarks was/weredetected (e.g., by the watermark detector 210) when performing watermarkdetection for the first monitoring time interval. In such examples, thereference response identifier 310 further identifies a first frequencyresponse determined by the frequency response determiner 305 for thefirst monitoring time interval to be a reference frequency responseindicative of the monitored media device being active, because the validwatermark(s) was(were) detected when performing watermark detection forthe first monitoring time interval and, thus, indicate that themonitored media device was active during the first time interval. Insuch examples, the reference response identifier 310 also stores theidentified, reference frequency response in an example reference storage315 for comparison with subsequent frequency responses determined by thefrequency response determiner 305. As mentioned above, in some examples,the reference response identifier 310 compares the energy of the sensedaudio signal to a threshold to verify that the first frequency responsedetermined for the first monitoring time interval corresponds to themonitored media device (and not some other media device at anotherlocation) before identifying the first frequency response to be a newreference frequency response for the monitored media device.

In some examples, the reference response identifier 310 also updates thereference frequency response stored in the reference storage 315 for amonitored media device. For example, the reference response identifier310 may identify a subsequent frequency response determined by thefrequency response determiner 305 for a subsequent monitoring timeinterval (which may correspond to one or more time intervals afteridentification of a prior reference frequency response) as a referencefrequency response for the monitored media device (e.g., which indicatesthe monitored media device is active) in response to determining thatone or more valid watermarks was/were detected (e.g., by the watermarkdetector 210) when performing watermark detection for the subsequentmonitoring time interval. In some examples, the reference responseidentifier 310 replaces the reference frequency response stored in thereference storage 315 for the monitored media device with the new,measured frequency response determined by the frequency responsedeterminer 305 for this subsequent monitoring time interval. In someexamples, the reference response identifier 310 identifies the new,measured frequency response as another reference frequency response tobe compared with subsequent measured frequency responses (e.g., inaddition to the prior reference frequency response(s) determined for themonitored media device). The example reference storage 315 may beimplemented by any appropriate memory, storage device, etc., such as oneor more of the volatile memory 1114 and/or the mass storage device 1128of the example processor platform 1100 of FIG. 11, which is described infurther detail below.

The example device activation detector 110 of FIG. 3 further includes anexample comparator 320 to compare the reference frequency responsedetermined by the frequency response determiner 305 for a first (e.g.,earlier) monitoring time interval with a measured frequency responsedetermined by the frequency response determiner 305 for a second (e.g.,later) monitoring time interval. In some examples, and as described infurther detail below, a frequency response determined by the frequencyresponse determiner 305 is represented as a vector of frequency responsevalues over a set of frequency indices. In such examples, the comparator320 is structured to compute a dot product, or some other correlationmetric, of the current measured frequency response (e.g., determined bythe frequency response determiner 305 for the second, e.g., later,monitoring time interval) with the reference frequency responseretrieved from the reference storage 315 (e.g., and which was determinedby the frequency response determiner 305 for the first, e.g., earlier,monitoring time interval). To compute the dot product, for eachfrequency index, the comparator 320 multiples the correspondingfrequency response values for the current measured frequency responseand the reference frequency response, and sums the multiplied values todetermine the dot product. In some examples, the comparator 320 thencompares the dot product result to a threshold to determine whether thecurrent measured frequency response and the reference frequency responsematch. For example, the comparator 320 may determine that the currentmeasured frequency response and the reference frequency response matchif the dot product result satisfies (e.g., meets or exceeds) thethreshold, and may determine that the current measured frequencyresponse and the reference frequency response do not match if the dotproduct result does not satisfy (e.g., does not meet or exceed) thethreshold.

In the illustrated example of FIG. 3, the device activation detector 110also includes an example status evaluator 325 to determine and reportstatus information associated with media device activity. For example,if the comparator 320 determines that the measured frequency responsefor a given monitoring time interval matches the reference frequencyresponse, then the status evaluator 325 indicates that the monitoredmedia device was active (turned on) during the given monitoring timeinterval. Conversely, if the comparator 320 determines that the measuredfrequency response for the given monitoring time interval did not matchthe reference frequency response, then the status evaluator 325indicates that the monitored media device was inactive (e.g., turnedoff) during the given monitoring time interval. In some examples, thestatus evaluator 325 further compares the energy of the sensed audiosignal to a threshold (and/or examines some other characteristic(s) ofthe sensed audio signal) to distinguish between the monitored mediadevice being inactive and the sensed audio signal being corrupted byanother audio source (e.g., ambient acoustic noise, people talking inthe vicinity of the monitored media device, audio emanating from someother media device at another location, etc.). For example, if thereference and measured frequency responses did not match and the energyof the audio signal does not satisfy (e.g., is less than or does notexceed) the threshold, then the weak audio signal indicates to thestatus evaluator 325 that the monitored media device was inactive (e.g.,turned off). However, if the reference and measured frequency responsesdid not match and the energy of the audio signal satisfies (e.g., meetsor exceeds) the threshold, then the strong audio signal indicates to thestatus evaluator 325 that the sensed audio signal was dominated by anaudio source other than the monitored media device and, as such, thestatus of the monitored media device is unknown.

In some examples, the status evaluator 325 further asserts or otherwiseoutputs a control signal, indication, etc., to condition whether furthermonitoring data, such as signatures, are to be generated for the givenmonitoring time interval based on the results of comparing the measuredfrequency response for the given monitoring time interval with thereference frequency response for the monitored media device. Forexample, if the comparator 320 determines that the measured frequencyresponse for the given monitoring time interval matches the referencefrequency response, then in addition to indicating that the monitoredmedia device was active (turned on), the status evaluator 325 may alsoassert a control signal, indication, etc., to permit signaturegeneration, such as by the example signature generator 215, to beperformed to generate signatures from the sensed audio signal for thegiven monitoring time interval. However, if the comparator 320determines that the measured frequency response for the given monitoringtime interval does not match the reference frequency response, then inaddition to indicating that the monitored media device was inactive(turned off), the status evaluator 325 may also assert a control signal,indication, etc., to prevent signature generation, such as by theexample signature generator 215, from being performed for the givenmonitoring time interval.

In some examples, when the reference response identifier 310 replaces aprior reference response (e.g., determined for a prior first monitoringtime interval) with a new reference frequency response determined for alater second, third, etc., time interval, the status evaluator 325 alsocauses the comparator 320 to compare the new reference frequencyresponse with the prior reference frequency response. For example, thecomparator 320 may compute the dot product of the two referencefrequency responses and compare the resulting dot product value to thethreshold. If the comparator 320 determines the two reference frequencyresponses match (e.g., their dot product satisfies the threshold), thenthe status evaluator 325 indicates that the environment of the monitoredsite (e.g., the monitored site 100) is unchanged. However, if thecomparator 320 determines the two reference frequency responses do notmatch (e.g., their dot product does not satisfy the threshold), then thestatus evaluator 325 indicates that a position of the monitored mediadevice has moved relative to a prior position, or an environment inwhich the media device is operating has changed relative to a priorarrangement of the environment, etc.

A block diagram illustrating an example implementation of the frequencyresponse determiner 305 of FIG. 3 is illustrated in FIG. 4. In theillustrated example of FIG. 4, the frequency response determiner 305 isstructured to determine a frequency response associated with a sensedaudio signal from frequency values used to perform watermark detection(e.g., by the watermark detector 210). Furthermore, the examplefrequency response determiner 305 of FIG. 4 is structured based on anassumption that the frequency values correspond to spectral power valuesof the sensed audio signal determined for frequency bins in thefrequency range of 3 kHz to 5.3 kHz from samples of the sensed audiosignal obtained during a given monitoring interval. Furthermore, thefrequency bins are assumed to be grouped into watermark code bandsconveying watermark data. In the illustrated example of FIG. 4, thefrequency response determiner 305 of FIG. 4 is structured assuming thatthe frequency values correspond to spectral power values determined for108 frequency bins grouped into 8 code bands in the frequency range of 3kHz to 5.3 kHz. Furthermore, the frequency bins in a code band areassumed to be separated from each other by 5.208 Hz. However, otherarrangements and groupings of frequency bins and code bands can besupported by the example frequency response determiner 305 of FIG. 4.

In the illustrated example, the frequency response determiner 305averages the frequency values over a monitoring time interval, such as10 seconds or some other time period. For example, if the frequencyvalues used to perform watermark detection are determined by thewatermark decoder 210 at 100 millisecond intervals, then averaging thefrequency values for each frequency bin over the monitoring timeinterval (e.g., 10 seconds) causes the averaged frequency values tobecome less dependent on the particular media being presented by themonitored media device and, instead, causes the averaged frequencyvalues to become more dependent on the multipath interference observedin the sensed audio. Accordingly, when the monitored media device isactive (e.g., turned on), the averaged frequency values will tend tohave a similar distribution across monitoring time interval, whereas theaveraged frequency values will tend to change when the monitored mediadevice is inactive (e.g., turned off) and the sensed audio signal isbased on audio source(s) other than the monitored media device.

With the foregoing in mind, the example frequency response determiner305 of FIG. 4 includes an example frequency band normalizer 405 tonormalize the spectral power values for the frequency bins based on theaverage power for the code bands in which the bins reside. For example,let P_(b,n)(k) represent the spectral power of a frequency bin havingbin index n, where n=0, 1, . . . , 8, in a code band b, where b=0, 1, .. . , 11, and observed at a time step k. To normalize the spectral powervalue of the frequency bin n in band b, the frequency band normalizer405 computes the average power in the band b by summing the individualspectral power values of the frequency bins in the band and dividing bythe number of bins in the band. Mathematically, the frequency bandnormalizer 405 computes the average power in the band b according toEquation 1, which is:

$\begin{matrix}{{P_{b,{avg}}(k)} = {\sum\limits_{n = 0}^{n = 8}\;{P_{b,n}(k)}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Next, the frequency band normalizer 405 computes the normalized spectralpower of frequency bin n in the band b by dividing the spectral power offrequency bin n by the average power in the band b determined accordingto Equation 1. Mathematically, the frequency band normalizer 405computes the normalized spectral power of frequency bin n in the band baccording to Equation 2, which is:

$\begin{matrix}{{P_{b,n}(k)} = \frac{P_{b,n}(k)}{{Pb},{avg}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

The frequency band normalizer 405 repeats the foregoing computations ofEquation 1 and Equation 2 for all the frequency bins n=0, 1, . . . , 8over all the code bands b=0, 1, . . . , 11.

The example frequency response determiner 305 of FIG. 4 also includes anexample frequency bin averager 410 to determine time averaged,normalized spectral power values for the for the different frequencybins normalize. For example, the frequency bin averager 410 computes thetime-averaged normalized power of bin n in code band b by summing thenormalized spectral power, p_(b,n)(k), of frequency bin n over themonitoring time interval. If the time step, k, corresponds to a timeshift of 100 ms, and the monitoring time interval is 10 seconds, thenthe frequency bin averager 410 of the illustrated example computes thetime-averaged normalized power of bin n in code band b mathematicallyaccording to Equation 3, which is:

$\begin{matrix}{p_{b,n,{tavg}} = {\sum\limits_{k = 0}^{99}\;{{p_{b,n}(k)}/100}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$In Equation 3, the summation is over 100 time steps, k, totaling 10seconds. The frequency bin averager 410 repeats the foregoingcomputations of Equation 3 for all the frequency bins n=0, 1, . . . , 8over all the code bands b=0, 1, . . . , 11.

In the illustrated example of FIG. 4, the example frequency responsedeterminer 305 further includes an example DC remover 415 and an examplefrequency bin normalizer 420 to convert the resulting distribution ofthe time-averaged, normalized spectral bin powers, p_(b,n,tavg), forb=0, 1, . . . , 11, and n=0, 1, . . . , 8, into a frequency responserepresented by a normalized vector of spectral power coefficients. Inthe illustrated example, as there are 12 code bands (b=0, 1, . . . , 11)and 9 frequency bins (n=0, 1, . . . , 8) in each code band, theresulting frequency response vector will contain 108 elements. In theillustrated example, the DC remover 415 determines the normalized vectorof spectral power coefficients, which forms the frequency response, byfirst computing the direct current (DC) component of the time-averaged,normalized spectral bin powers. Mathematically, the DC remover 415determines the DC component according to Equation 4, which is:

$\begin{matrix}{p_{dc} = {\sum\limits_{b = 0}^{b = 11}\;{\sum\limits_{n = 0}^{n = 8}\; p_{b,n,{tavg}}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

The DC remover 415 then removes the DC component, p_(dc), from thetime-averaged, normalized spectral bin powers by subtraction to yieldjust the alternating current (AC) components of the time-averaged,normalized spectral bin powers. Mathematically, the DC remover 415removes the DC component from the time-averaged, normalized spectral binpower, p_(b,n,tavg), according to Equation 5, which is:p _(b,n,ac) =p _(b,n,tavg) −p _(dc)   Equation 5

Then, the frequency bin normalizer 420 of the illustrated exampledetermines the spectral power coefficient for each frequency bin bydividing the time-averaged, AC spectral power of the frequency bins,p_(b,n,ac), by the normalized power across all bins, i.e.p_(norm)=Σ_(b=0) ^(b=11)Σ₀ ⁸p_(b,n,ac) ². Mathematically, the spectralpower coefficient for frequency bin n in band b is represented byp_(b,n,resp) (as the spectral power coefficient corresponds to thefrequency response determined for that bin and band), and is determinedby the frequency bin normalizer 420 according to Equation 6, which is

$\begin{matrix}{p_{b,n,{resp}} = \frac{p_{b,n,{ac}}}{p_{norm}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

The DC remover 415 and the frequency bin normalizer 420 repeat theforegoing computations of Equation 4 through Equation 6 for all thefrequency bins n=0, 1, . . . 8 over all the code bands b=0, 1, . . . ,11. In the illustrated example of FIG. 4, the spectral powercoefficients, p_(b,n,resp), determined for the frequency bins n=0, 1, .. . , 8 over the code bands b=0, 1, . . . 11 form the frequency responsedetermined using frequency values the P_(b,n)(k) determined and used forwatermark detection during the current monitoring time interval. Ifwatermark detection yields one or more valid watermarks for the currentmonitoring time interval, the resulting frequency response becomes areference frequency response. Otherwise, the frequency responsecorresponds to a measured frequency response that is to be compared witha previously determined reference frequency response to determinewhether the monitored media device is active.

Example frequency responses 505 and 510 that may be determined by theexample frequency response determiner 305 of FIGS. 3 and/or 4 areillustrated in FIGS. 5A and 5B, respectively. The example frequencyresponse 505 corresponds to a frequency response determined by theexample frequency response determiner 305 when the monitored mediadevice 115 is active (e.g., turned on). In contrast, the examplefrequency response 510 corresponds to a frequency response determined bythe example frequency response determiner 305 when the monitored mediadevice 115 is inactive (e.g., turned off). As noted above, the exampledevice activation detector 110 may determine that the example frequencyresponse 505 corresponds to the monitored media device being active inresponse to determining that one or more valid watermarks were detectedby the example watermark detector 210 during the monitoring timeinterval for which the frequency response 505 was computed. Asillustrated in FIGS. 5A and 5B, the example frequency response 505 and510 exhibit substantially different patterns in their respectivespectral coefficients. Accordingly, a dot product of these two frequencyresponses, as determined by the example comparator 320, would likelyyield a negative value having a magnitude lower than 0.5. Thus, in someexamples, the comparator 320 employs a threshold of 0.5 (or some othervalue) to determine whether the dot product of two frequency responsevectors indicates that the responses match.

While example manners of implementing the meter 105 and the deviceactivation detector 110 are illustrated in FIGS. 1-4, one or more of theelements, processes and/or devices illustrated in FIGS. 1-4 may becombined, divided, re-arranged, omitted, eliminated and/or implementedin any other way. Further, the example microphone 120, the example audiosignal sampler 205, the example watermark detector 210, the examplesignature generator 215, the example data reporter 220, the examplenetwork 225, the example data processing facility 230, the examplefrequency response determiner 305, the example reference responseidentifier 310, the example reference storage 315, the examplecomparator 320, the example status evaluator 325, the example frequencyband normalizer 405, the example frequency bin averager 410, the exampleDC remover 415, the example frequency bin normalizer 420 and/or, moregenerally, the example meter 105 and/or the example device activationdetector 110 of FIGS. 1-4 may be implemented by hardware, software,firmware and/or any combination of hardware, software and/or firmware.Thus, for example, any of the example microphone 120, the example audiosignal sampler 205, the example watermark detector 210, the examplesignature generator 215, the example data reporter 220, the examplenetwork 225, the example data processing facility 230, the examplefrequency response determiner 305, the example reference responseidentifier 310, the example reference storage 315, the examplecomparator 320, the example status evaluator 325, the example frequencyband normalizer 405, the example frequency bin averager 410, the exampleDC remover 415, the example frequency bin normalizer 420 and/or, moregenerally, the example meter 105 and/or the example device activationdetector 110 could be implemented by one or more analog or digitalcircuit(s), logic circuits, programmable processor(s), applicationspecific integrated circuit(s) (ASIC(s)), programmable logic device(s)(PLD(s)) and/or field programmable logic device(s) (FPLD(s)). Whenreading any of the apparatus or system claims of this patent to cover apurely software and/or firmware implementation, at least one of theexample meter 105, the example device activation detector 110, theexample microphone 120, the example audio signal sampler 205, theexample watermark detector 210, the example signature generator 215, theexample data reporter 220, the example network 225, the example dataprocessing facility 230, the example frequency response determiner 305,the example reference response identifier 310, the example referencestorage 315, the example comparator 320, the example status evaluator325, the example frequency band normalizer 405, the example frequencybin averager 410, the example DC remover 415 and/or the examplefrequency bin normalizer 420 is/are hereby expressly defined to includea tangible computer readable storage device or storage disk such as amemory, a digital versatile disk (DVD), a compact disk (CD), a Blu-raydisk, etc. storing the software and/or firmware. Further still, theexample meter 105 and/or the device activation detector 110 may includeone or more elements, processes and/or devices in addition to, orinstead of, those illustrated in FIGS. 1-4, and/or may include more thanone of any or all of the illustrated elements, processes and devices.

Flowcharts representative of example machine readable instructions forimplementing the example meter 105, the example device activationdetector 110, the example microphone 120, the example audio signalsampler 205, the example watermark detector 210, the example signaturegenerator 215, the example data reporter 220, the example network 225,the example data processing facility 230, the example frequency responsedeterminer 305, the example reference response identifier 310, theexample reference storage 315, the example comparator 320, the examplestatus evaluator 325, the example frequency band normalizer 405, theexample frequency bin averager 410, the example DC remover 415 and/orthe example frequency bin normalizer 420 are shown in FIGS. 6-9. Inthese examples, the machine readable instructions comprise one or moreprograms for execution by a processor, such as the processors 1012, 1112and/or 1212 shown in the example processor platforms 1000, 1100 and/or1200 discussed below in connection with FIGS. 10-12. The one or moreprograms, or portion(s) thereof, may be embodied in software stored on atangible computer readable storage medium such as a CD-ROM, a floppydisk, a hard drive, a digital versatile disk (DVD), a Blu-ray Disk™ or amemory associated with the processor 1012, 1112 and/or 1212, but theentire program or programs and/or portions thereof could alternativelybe executed by a device other than the processor 1012, 1112 and/or 1212,and/or embodied in firmware or dedicated hardware (e.g., implemented byan ASIC, a PLD, an FPLD, discrete logic, etc.). Further, although theexample program(s) is(are) described with reference to the flowchartsillustrated in FIGS. 6-9, many other methods of implementing the examplemeter 105, the example device activation detector 110, the examplemicrophone 120, the example audio signal sampler 205, the examplewatermark detector 210, the example signature generator 215, the exampledata reporter 220, the example network 225, the example data processingfacility 230, the example frequency response determiner 305, the examplereference response identifier 310, the example reference storage 315,the example comparator 320, the example status evaluator 325, theexample frequency band normalizer 405, the example frequency binaverager 410, the example DC remover 415 and/or the example frequencybin normalizer 420 may alternatively be used. For example, withreference to the flowcharts illustrated in FIGS. 6-9, the order ofexecution of the blocks may be changed, and/or some of the blocksdescribed may be changed, eliminated, combined and/or subdivided intomultiple blocks.

As mentioned above, the example processes of FIGS. 6-9 may beimplemented using coded instructions (e.g., computer and/or machinereadable instructions) stored on a tangible computer readable storagemedium such as a hard disk drive, a flash memory, a read-only memory(ROM), a compact disk (CD), a digital versatile disk (DVD), a cache, arandom-access memory (RAM) and/or any other storage device or storagedisk in which information is stored for any duration (e.g., for extendedtime periods, permanently, for brief instances, for temporarilybuffering, and/or for caching of the information). As used herein, theterm tangible computer readable storage medium is expressly defined toinclude any type of computer readable storage device and/or storage diskand to exclude propagating signals and to exclude transmission media. Asused herein, “tangible computer readable storage medium” and “tangiblemachine readable storage medium” are used interchangeably. Additionallyor alternatively, the example processes of FIGS. 6-9 may be implementedusing coded instructions (e.g., computer and/or machine readableinstructions) stored on a non-transitory computer and/or machinereadable medium such as a hard disk drive, a flash memory, a ROM, a CD,a DVD, a cache, a RAM and/or any other storage device or storage disk inwhich information is stored for any duration (e.g., for extended timeperiods, permanently, for brief instances, for temporarily buffering,and/or for caching of the information). As used herein, the termnon-transitory computer readable medium is expressly defined to includeany type of computer readable storage device and/or storage disk and toexclude propagating signals and to exclude transmission media. As usedherein, when the phrase “at least” is used as the transition term in apreamble of a claim, it is open-ended in the same manner as the term“comprising” is open ended. Also, as used herein, the terms “computerreadable” and “machine readable” are considered equivalent unlessindicated otherwise.

An example program 600 that may be executed to implement the exampledevice meter 105 of FIGS. 1 and/or 2 is represented by the flowchartshown in FIG. 6. For convenience and without loss of generality,execution of the example program 600 is described from the perspectiveof the example device meter 105 of FIG. 2 operating in the examplemonitored site 100 of FIG. 1. With reference to the preceding figuresand associated written descriptions, the example program 600 of FIG. 6begins execution at block 605 at which the example audio signal sampler205 of the device meter 105 samples the audio signal output from themicrophone 120 for a current monitoring time interval. At block 610, theexample watermark detector 210 of the device meter 105 performswatermark detection for the current monitoring time interval using theaudio signal samples obtained at block 605. At block 615, the exampledevice activation detector 110 of the device meter 105 performs deviceactivity detection using frequency values determined at block 610 forwatermark detection to determine whether a monitored media device (e.g.,the media device 115) was active during the current monitoring timeinterval. An example program that may be executed to perform theprocessing at block 615 is illustrated in FIG. 7 and described infurther detail below.

At block 620, the device activation detector 110 determines whether thefollowing two conditions were met for the current monitoring timeinterval: (1) no valid watermark was detected at block 610 and (2) themonitored media device was determined to be active at block 615. If bothconditions are met (block 620), then processing proceeds to block 625 atwhich the example signature generator 215 of the device meter 105generates signatures from the audio samples obtained at block 605.Otherwise, if both condition examined at block 620 are not met, or whenprocessing completes at block 625, processing then proceeds to block 630at which the example data reporter 220 of the device meter 105 storesthe metering data determined by the device meter 105 for the currentmonitoring time interval for reporting to the example data processingfacility 230. For example, at block 630 the data reporter 220 stores thewatermark(s), if any, decoded at block 610, the signature(s), if any,generated at block 625, and/or the device activity determinations and/orother status information determined at block 615. At block 635, thedevice meter 105 determines whether monitoring of the monitored mediadevice is to continue. If monitoring is to continue, processing returnsto block 605 and blocks subsequent thereto to perform monitoring for thenext monitoring time interval. Otherwise, execution of the exampleprogram 600 ends.

An example program 615P that may be executed to implement the exampledevice activation detector 110 of FIGS. 1, 2 and/or 3, and/or that maybe used to perform the processing at block 615 of FIG. 6, is representedby the flowchart shown in FIG. 7. For convenience and without loss ofgenerality, execution of the example program 615P is described from theperspective of the example device activation detector 110 of FIG. 3being included in the example device meter 105 of FIG. 2 and operatingin the example monitored site 100 of FIG. 1. With reference to thepreceding figures and associated written descriptions, the exampleprogram 615P of FIG. 7 begins execution at block 705 at which theexample frequency response determiner 305 of the device activationdetector 110 accesses the frequency values (e.g., the values P_(b,n)(k)describe above) determined by the watermark detector 210 for performingwatermark detection for the current monitoring time interval. At block710, the frequency response determiner 305 processes the frequencyvalues obtained at block 705 to determine a measured frequency response(e.g., corresponding to the vector of the spectral power coefficients,p_(b,n,resp), described above) of the sensed audio signal for thecurrent monitoring interval. An example program that may be executed toperform the processing at block 710 is illustrated in FIG. 9 anddescribed in further detail below.

At block 715, the example reference response identifier 310 of thedevice activation detector 110 determines whether the media device isactive (e.g., turned on) by, for example, determining whether validwatermarks have been decoded by the watermark detector 210 for thecurrent monitoring time interval. If the media device is determined tobe active (block 715), then at block 720 the reference responseidentifier 310 stores the measured frequency response determined atblock 710 in the example reference storage 315 as the latest referencefrequency response corresponding to the monitored media device beingactive. At block 725, the example status evaluator 325 of the deviceactivation detector 110 causes the comparator 320 to compare the currentreference frequency response with the prior reference frequencyresponse. If the comparator 320 determines the frequency responses match(block 725), then the monitored environment has not changed andexecution of the example program 615P ends. Otherwise, if the comparator320 determines that the current and prior reference frequency responsesdo not match (block 725), then at block 730 the status evaluator 325indicates that (1) the position of the monitored media device haschanged since generation of the prior reference frequency spectrum,and/or (2) the arrangement of the environment at the monitored site 100has changed since generation of the prior reference frequency spectrum.Execution of the example program 615P then ends.

Returning to block 715, if reference response identifier 310 is unableto determine that the monitored media device is active during thecurrent monitoring time interval (e.g., because no valid watermarks havebeen detected), then at block 735 the comparator 320 compares themeasured frequency response determined at block 710 with the latestreference frequency response indicative of the monitored media devicebeing active. If at block 740 the comparator 320 determines that themeasured frequency response and the reference frequency response match(e.g., because their dot product satisfies a threshold), then at block745 the status evaluator 325 indicates that the monitored media device(e.g., the media device 115) was active (e.g., turned on) for thecurrent monitoring time interval, and permits further media devicemonitoring to be performed for at least the remainder of the currentmonitoring time interval. An example program that may be executed toperform the processing at block 745 is illustrated in FIG. 8 anddescribed in further detail below. Otherwise, if the comparator 320determines that the measured frequency response and the referencefrequency response do not match (e.g., because their dot product doesnot satisfy a threshold), then at block 750 the status evaluator 325indicates that the monitored media device (e.g., the media device 115)was inactive (e.g., turned off) for the current monitoring timeinterval, and disables further media device monitoring for at least theremainder of the current monitoring time interval. Execution of theexample program 615P then ends.

An example program 745P that may be used to perform the processing atblock 745 of FIG. 7 is represented by the flowchart shown in FIG. 8.With reference to the preceding figures and associated writtendescriptions, the example program 745P of FIG. 8 begins execution atblock 805 at which the comparator 320 determines that the measuredfrequency response and the reference frequency response match (e.g.,because their dot product satisfies a threshold). At block 810, thestatus evaluator 325 indicates that the monitored media device (e.g.,the media device 115) was active during the current monitoring timeinterval (e.g., because the measured frequency device was determined tomatch the reference frequency response indicative of the monitored mediadevice being active). In some examples, at block 815, the statusevaluator 325 indicates that the monitored environment at the monitoredsite 100 remains unchanged relative to the prior monitoring timeinterval. At block 820 the status evaluator 325 further asserts orotherwise outputs a control signal, indication, etc., to enable thesignature generator 215 to generate signatures from the sensed audiosignal for the current monitoring interval. Execution of the exampleprogram 745P then ends.

An example program 710P that may be executed to implement the examplefrequency response determiner 305 of FIGS. 3 and/or 4, and/or that maybe used to perform the processing at block 710 of FIG. 7, is representedby the flowchart shown in FIG. 9. For convenience and without loss ofgenerality, execution of the example program 710P is described from theperspective of the example frequency response determiner 305 of FIG. 4being included in the example device activation detector 110 of FIG. 3.With reference to the preceding figures and associated writtendescriptions, the example program 710P of FIG. 9 begins execution atblocks 905 and 910 at which the frequency response determiner 305 beginsiterating through the frequency bins and the watermark code bandsdefined for the frequency spectrum of the sensed audio signal. Forexample, at block 915, the example frequency band normalizer 405 of thefrequency response determiner 305 normalizes (e.g., using Equation 1 andEquation 2 as disclosed above) the frequency values, namely, thespectral power values, P_(b,n)(k), to determine the normalized spectralpower, p_(b,n)(k), for the frequency bin n in the watermark band bcorresponding to the current processing iteration. At block 920, theexample frequency bin averager 410 of the frequency response determiner305 averages (e.g., using Equation 3 as disclosed above) the normalizedspectral power, p_(b,n)(k), over time, k, for the frequency bin n in thewatermark band b to determine the time-averaged normalized power,p_(b,n,tavg), for the frequency bin n in the watermark band bcorresponding to the current processing iteration. Iteration thencontinues at block 925 and 930 until values of the time-averagednormalized power, p_(b,n,tavg), are determined for all frequency bins nin all watermark bands b.

Next, at block 935 the example DC remover 415 of the frequency responsedeterminer 305 removes (e.g., using Equation 4 and Equation 5, asdisclosed above) the DC component from the time-averaged normalizedpower, p_(b,n,tavg), to determine the time-averaged, AC spectral power,p_(b,n,ac), of frequency bin n in watermark band b. At block 940, theexample frequency bin normalizer 420 of the frequency responsedeterminer 305 normalizes (e.g., using Equation 6, as disclosed above)the time-averaged, AC spectral powers, p_(b,n,ac), frequency of the binsn in the watermark bands b to determine the spectral power coefficients,p_(b,n,resp), for the bins n in the bands b, which correspond to thefrequency response of the sensed audio signal determined for the currentmonitoring time interval. Execution of the example program 710P thenends.

FIG. 10 is a block diagram of an example processor platform 1000 capableof executing the instructions of FIG. 6 to implement the example meter105 of FIGS. 1 and/or 2. The processor platform 1000 can be, forexample, a server, a personal computer, a mobile device (e.g., a cellphone, a smart phone, a tablet such as an iPad™), a personal digitalassistant (PDA), an Internet appliance, a DVD player, a CD player, adigital video recorder, a Blu-ray player, a gaming console, a personalvideo recorder, a set top box a digital camera, or any other type ofcomputing device.

The processor platform 1000 of the illustrated example includes aprocessor 1012. The processor 1012 of the illustrated example ishardware. For example, the processor 1012 can be implemented by one ormore integrated circuits, logic circuits, microprocessors or controllersfrom any desired family or manufacturer. In the illustrated example ofFIG. 10, the processor 1012 is configured via example instructions 1032to implement the example meter 105, the example device activationdetector 110, the example microphone 120, the example audio signalsampler 205, the example watermark detector 210, the example signaturegenerator 215, the example data reporter 220, the example network 225,and/or the example data processing facility 230 of FIGS. 1 and/or 2.

The processor 1012 of the illustrated example includes a local memory1013 (e.g., a cache). The processor 1012 of the illustrated example isin communication with a main memory including a volatile memory 1014 anda non-volatile memory 1016 via a link 1018. The link 1018 may beimplemented by a bus, one or more point-to-point connections, etc., or acombination thereof. The volatile memory 1014 may be implemented bySynchronous Dynamic Random Access Memory (SDRAM), Dynamic Random AccessMemory (DRAM), RAMBUS Dynamic Random Access Memory (RDRAM) and/or anyother type of random access memory device. The non-volatile memory 1016may be implemented by flash memory and/or any other desired type ofmemory device. Access to the main memory 1014, 1016 is controlled by amemory controller.

The processor platform 1000 of the illustrated example also includes aninterface circuit 1020. The interface circuit 1020 may be implemented byany type of interface standard, such as an Ethernet interface, auniversal serial bus (USB), and/or a PCI express interface.

In the illustrated example, one or more input devices 1022 are connectedto the interface circuit 1020. The input device(s) 1022 permit(s) a userto enter data and commands into the processor 1012. The input device(s)can be implemented by, for example, an audio sensor, a microphone, acamera (still or video), a keyboard, a button, a mouse, a touchscreen, atrack-pad, a trackball, a trackbar (such as an isopoint), a voicerecognition system and/or any other human-machine interface. Also, manysystems, such as the processor platform 1000, can allow the user tocontrol the computer system and provide data to the computer usingphysical gestures, such as, but not limited to, hand or body movements,facial expressions, and face recognition. In the illustrated example ofFIG. 10, the input device(s) 1022 include the example microphone 120.

One or more output devices 1024 are also connected to the interfacecircuit 1020 of the illustrated example. The output devices 1024 can beimplemented, for example, by display devices (e.g., a light emittingdiode (LED), an organic light emitting diode (OLED), a liquid crystaldisplay, a cathode ray tube display (CRT), a touchscreen, a tactileoutput device, a printer and/or speakers). The interface circuit 1020 ofthe illustrated example, thus, typically includes a graphics drivercard, a graphics driver chip or a graphics driver processor.

The interface circuit 1020 of the illustrated example also includes acommunication device such as a transmitter, a receiver, a transceiver, amodem and/or network interface card to facilitate exchange of data withexternal machines (e.g., computing devices of any kind) via a network1026 (e.g., an Ethernet connection, a digital subscriber line (DSL), atelephone line, coaxial cable, a cellular telephone system, etc.).

The processor platform 1000 of the illustrated example also includes oneor more mass storage devices 1028 for storing software and/or data.Examples of such mass storage devices 1028 include floppy disk drives,hard drive disks, compact disk drives, Blu-ray disk drives, RAID(redundant array of independent disks) systems, and digital versatiledisk (DVD) drives.

Coded instructions 1032 corresponding to the instructions of FIG. 6 maybe stored in the mass storage device 1028, in the volatile memory 1014,in the non-volatile memory 1016, in the local memory 1013 and/or on aremovable tangible computer readable storage medium, such as a CD or DVD1036.

FIG. 11 is a block diagram of an example processor platform 1100 capableof executing the instructions of FIGS. 7 and/or 8 to implement theexample device activation detector 110 of FIGS. 1, 2 and/or 3. Theprocessor platform 1100 can be, for example, a server, a personalcomputer, a mobile device (e.g., a cell phone, a smart phone, a tabletsuch as an iPad™), a PDA, an Internet appliance, a DVD player, a CDplayer, a digital video recorder, a Blu-ray player, a gaming console, apersonal video recorder, a set top box a digital camera, or any othertype of computing device.

The processor platform 1100 of the illustrated example includes aprocessor 1112. The processor 1112 of the illustrated example ishardware. For example, the processor 1112 can be implemented by one ormore integrated circuits, logic circuits, microprocessors or controllersfrom any desired family or manufacturer. In the illustrated example ofFIG. 11, the processor 1112 is configured via example instructions 1132to implement the example device activation detector 110, the examplefrequency response determiner 305, the example reference responseidentifier 310, the example comparator 320 and/or the example statusevaluator 325 of FIGS. 1, 2 and/or 3.

The processor 1112 of the illustrated example includes a local memory1113 (e.g., a cache). The processor 1112 of the illustrated example isin communication with a main memory including a volatile memory 1114 anda non-volatile memory 1116 via a link 1118. The link 1118 may beimplemented by a bus, one or more point-to-point connections, etc., or acombination thereof. The volatile memory 1114 may be implemented bySDRAM, DRAM, RDRAM and/or any other type of random access memory device.The non-volatile memory 1116 may be implemented by flash memory and/orany other desired type of memory device. Access to the main memory 1114,1116 is controlled by a memory controller.

The processor platform 1100 of the illustrated example also includes aninterface circuit 1120. The interface circuit 1120 may be implemented byany type of interface standard, such as an Ethernet interface, a USB,and/or a PCI express interface.

In the illustrated example, one or more input devices 1122 are connectedto the interface circuit 1120. The input device(s) 1122 permit(s) a userto enter data and commands into the processor 1112. The input device(s)can be implemented by, for example, an audio sensor, a microphone, acamera (still or video), a keyboard, a button, a mouse, a touchscreen, atrack-pad, a trackball, a trackbar (such as an isopoint), a voicerecognition system and/or any other human-machine interface. Also, manysystems, such as the processor platform 1100, can allow the user tocontrol the computer system and provide data to the computer usingphysical gestures, such as, but not limited to, hand or body movements,facial expressions, and face recognition.

One or more output devices 1124 are also connected to the interfacecircuit 1120 of the illustrated example. The output devices 1124 can beimplemented, for example, by display devices (e.g., an LED, an OLED, aliquid crystal display, a CRT, a touchscreen, a tactile output device, aprinter and/or speakers). The interface circuit 1120 of the illustratedexample, thus, typically includes a graphics driver card, a graphicsdriver chip or a graphics driver processor.

The interface circuit 1120 of the illustrated example also includes acommunication device such as a transmitter, a receiver, a transceiver, amodem and/or network interface card to facilitate exchange of data withexternal machines (e.g., computing devices of any kind) via a network1126 (e.g., an Ethernet connection, a DSL, a telephone line, coaxialcable, a cellular telephone system, etc.).

The processor platform 1100 of the illustrated example also includes oneor more mass storage devices 1128 for storing software and/or data.Examples of such mass storage devices 1128 include floppy disk drives,hard drive disks, compact disk drives, Blu-ray disk drives, RAIDsystems, and DVD drives. In some examples, the mass storage device 1128may implement the example reference storage 315. Additionally oralternatively, in some examples the volatile memory 1114 may implementthe example reference storage 315.

Coded instructions 1132 corresponding to the instructions of FIGS. 7and/or 8 may be stored in the mass storage device 1128, in the volatilememory 1114, in the non-volatile memory 1116, in the local memory 1113and/or on a removable tangible computer readable storage medium, such asa CD or DVD 1136.

FIG. 12 is a block diagram of an example processor platform 1200 capableof executing the instructions of FIG. 9 to implement the examplefrequency response determiner 305 of FIGS. 3 and/or 4. The processorplatform 1200 can be, for example, a server, a personal computer, amobile device (e.g., a cell phone, a smart phone, a tablet such as aniPad™), a PDA, an Internet appliance, a DVD player, a CD player, adigital video recorder, a Blu-ray player, a gaming console, a personalvideo recorder, a set top box a digital camera, or any other type ofcomputing device.

The processor platform 1200 of the illustrated example includes aprocessor 1212. The processor 1212 of the illustrated example ishardware. For example, the processor 1212 can be implemented by one ormore integrated circuits, logic circuits, microprocessors or controllersfrom any desired family or manufacturer. In the illustrated example ofFIG. 12, the processor 1212 is configured via example instructions 1232to implement the example frequency response determiner 305, the examplefrequency band normalizer 405, the example frequency bin averager 410,the example DC remover 415 and/or the example frequency bin normalizer420 of FIGS. 3 and/or 4.

The processor 1212 of the illustrated example includes a local memory1213 (e.g., a cache). The processor 1212 of the illustrated example isin communication with a main memory including a volatile memory 1214 anda non-volatile memory 1216 via a link 1218. The link 1218 may beimplemented by a bus, one or more point-to-point connections, etc., or acombination thereof. The volatile memory 1214 may be implemented bySDRAM, DRAM, RDRAM and/or any other type of random access memory device.The non-volatile memory 1216 may be implemented by flash memory and/orany other desired type of memory device. Access to the main memory 1214,1216 is controlled by a memory controller.

The processor platform 1200 of the illustrated example also includes aninterface circuit 1220. The interface circuit 1220 may be implemented byany type of interface standard, such as an Ethernet interface, a USB,and/or a PCI express interface.

In the illustrated example, one or more input devices 1222 are connectedto the interface circuit 1220. The input device(s) 1222 permit(s) a userto enter data and commands into the processor 1212. The input device(s)can be implemented by, for example, an audio sensor, a microphone, acamera (still or video), a keyboard, a button, a mouse, a touchscreen, atrack-pad, a trackball, a trackbar (such as an isopoint), a voicerecognition system and/or any other human-machine interface. Also, manysystems, such as the processor platform 1200, can allow the user tocontrol the computer system and provide data to the computer usingphysical gestures, such as, but not limited to, hand or body movements,facial expressions, and face recognition.

One or more output devices 1224 are also connected to the interfacecircuit 1220 of the illustrated example. The output devices 1224 can beimplemented, for example, by display devices (e.g., an LED, an OLED, aliquid crystal display, a CRT, a touchscreen, a tactile output device, aprinter and/or speakers). The interface circuit 1220 of the illustratedexample, thus, typically includes a graphics driver card, a graphicsdriver chip or a graphics driver processor.

The interface circuit 1220 of the illustrated example also includes acommunication device such as a transmitter, a receiver, a transceiver, amodem and/or network interface card to facilitate exchange of data withexternal machines (e.g., computing devices of any kind) via a network1226 (e.g., an Ethernet connection, a DSL, a telephone line, coaxialcable, a cellular telephone system, etc.).

The processor platform 1200 of the illustrated example also includes oneor more mass storage devices 1228 for storing software and/or data.Examples of such mass storage devices 1228 include floppy disk drives,hard drive disks, compact disk drives, Blu-ray disk drives, RAIDsystems, and digital versatile disk (DVD) drives.

Coded instructions 1232 corresponding to the instructions of FIG. 9 maybe stored in the mass storage device 1228, in the volatile memory 1214,in the non-volatile memory 1216, in the local memory 1213 and/or on aremovable tangible computer readable storage medium, such as a CD or DVD1236.

Although certain example methods, apparatus and articles of manufacturehave been disclosed herein, the scope of coverage of this patent is notlimited thereto. On the contrary, this patent covers all methods,apparatus and articles of manufacture fairly falling within the scope ofthe claims of this patent.

What is claimed is:
 1. A meter to monitor a media device, the meter comprising: a microphone to sense audio; a device activation detector to: reuse first frequency values of the sensed audio to determine a first frequency response of the sensed audio, the first frequency values having been determined to perform watermark detection during a first monitoring time interval; compare the first frequency response to a reference frequency response to determine whether the media device was active during the first monitoring time interval; and control operation of the meter based on the determination of whether the media device was active during the first monitoring time interval; and a data reporter to output a device activity determination indicating whether the media device was active during the first monitoring time interval.
 2. The meter of claim 1, wherein the device activation detector is further to determine the reference frequency response based on second frequency values of the sensed audio that were used to detect a valid watermark during a second monitoring time interval prior to the first monitoring time interval.
 3. The meter of claim 1, wherein, when a valid watermark is detected during the first monitoring time interval, the device activation detector is further to: indicate an environment including the media device is unchanged when the first frequency response matches the reference frequency response; and indicate the environment including the media device has changed when the first frequency does not match the reference frequency response.
 4. The meter of claim 3, wherein, when the first frequency response does not match the reference frequency response, the device activation detector is further to: replace the reference frequency response with the first frequency response when a signal strength of the sensed audio satisfies a threshold; and discard the valid watermark and the first frequency response when the signal strength of the sensed audio does not satisfy the threshold.
 5. The meter of claim 3, wherein the device activation detector is to determine whether the first frequency response matches the reference frequency response by: computing a dot product of the first frequency response and the reference frequency response; determining that the first frequency response matches the reference frequency response when the dot product satisfies a threshold; and determining that the first frequency response does not match the reference frequency response when the dot product does not satisfy the threshold.
 6. The meter of claim 1, wherein the device activation detector is to: determine the media device was active during the first monitoring time interval when the first frequency response matches the reference frequency response; and determine the media device was inactive during the first monitoring time interval when the first frequency response does not match the reference frequency response and a valid watermark is not detected during the first monitoring time interval.
 7. The meter of claim 1, wherein the device activity determination is a first device activity determination, and the data reporter is to report metering data to a data processing facility via a network, the metering data including watermarks detected by the watermark detector and device activity determinations made by the device activation detector, the device activity determinations including the first device activity determination.
 8. A method to monitor a media device with a meter, the method comprising: reusing first frequency values of audio sensed with a microphone to determine, by executing an instruction with a processor, a first frequency response of the sensed audio, the first frequency values having been determined to perform watermark detection during a first monitoring time interval; comparing, by executing an instruction with the processor, the first frequency response to a reference frequency response to determine whether the media device was active during the first monitoring time interval; controlling, by executing an instruction with the processor, operation of the meter based on the determination of whether the media device was active during the first monitoring time interval; and outputting, by executing an instruction with the processor, a device activity determination indicating whether the media device was active during the first monitoring time interval.
 9. The method of claim 8, further including determining the reference frequency response based on second frequency values of the sensed audio that were used to detect a valid watermark during a second monitoring time interval prior to the first monitoring time interval.
 10. The method of claim 8, further including: detecting a valid watermark during the first monitoring time interval; indicating an environment including the media device is unchanged in response to determining the first frequency response matches the reference frequency response; and indicating the environment including the media device has changed in response to determining the first frequency does not match the reference frequency response.
 11. The method of claim 10, further including, in response to determining the first frequency does not match the reference frequency response: comparing a signal strength of the sensed audio to a threshold; replacing the reference frequency response with the first frequency response when the signal strength of the sensed audio satisfies the threshold; and discarding the valid watermark and the first frequency response when the signal strength of the sensed audio does not satisfy the threshold.
 12. The method of claim 10, wherein the determining of whether the first frequency response matches the reference frequency response includes: computing a dot product of the first frequency response and the reference frequency response; determining that the first frequency response matches the reference frequency response when the dot product satisfies a threshold; and determining that the first frequency response does not match the reference frequency response when the dot product does not satisfy the threshold.
 13. The method of claim 8, further including: determining the media device was active during the first monitoring time interval when the first frequency response matches the reference frequency response; and determining the media device was inactive during the first monitoring time interval when the first frequency response does not match the reference frequency response and a valid watermark was not detected during the first monitoring time interval.
 14. The method of claim 8, further including: not detecting a valid watermark during the first monitoring time interval; enabling signature generation during the first monitoring time interval in response to determining the first frequency response matches the reference frequency response; and disabling signature generation during the first monitoring time interval in response to determining the first frequency does not match the reference frequency response.
 15. A non-transitory computer readable medium comprising computer readable instructions that, when executed by a processor of a meter, cause the processor to at least: reuse first frequency values of audio sensed with a microphone to determine a first frequency response of the sensed audio, the first frequency values having been determined to perform watermark detection during a first monitoring time interval; compare the first frequency response to a reference frequency response to determine whether a media device was active during the first monitoring time interval; control operation of the meter based on the determination of whether the media device was active during the first monitoring time interval; and output a device activity determination indicating whether the media device was active during the first monitoring time interval.
 16. The computer readable medium of claim 15, wherein the instructions, when executed, further cause the processor to determine the reference frequency response based on second frequency values of the sensed audio that were used to detect a valid watermark during a second monitoring time interval prior to the first monitoring time interval.
 17. The computer readable medium of claim 15, wherein the instructions, when executed, further cause the processor to: detect a valid watermark during the first monitoring time interval; indicate an environment including the media device is unchanged when the first frequency response matches the reference frequency response; and indicate the environment including the media device has changed when the first frequency does not match the reference frequency response.
 18. The computer readable medium of claim 17, wherein, when the first frequency does not match the reference frequency response, the instructions, when executed, further cause the processor to: compare a signal strength of the sensed audio to a threshold; replace the reference frequency response with the first frequency response when the signal strength of the sensed audio satisfies the threshold; and discard the valid watermark and the first frequency response when the signal strength of the sensed audio does not satisfy the threshold.
 19. The computer readable medium of claim 15, wherein the instructions, when executed, further cause the processor to: determine the media device was active during the first monitoring time interval when the first frequency response matches the reference frequency response; and determine the media device was inactive during the first monitoring time interval when the first frequency response does not match the reference frequency response and a valid watermark was not detected during the first monitoring time interval.
 20. The computer readable medium of claim 15, wherein, when a valid watermark is not detected during the first monitoring time interval, the instructions, when executed, further cause the processor to: enable signature generation during the first monitoring time interval in response to determining the first frequency response matches the reference frequency response; and disable signature generation during the first monitoring time interval in response to determining the first frequency does not match the reference frequency response. 